Chapter 14 : How Biofilms Evade Host Defenses

Abstract:

Historically, microbial organisms have been grown in pure liquid cultures as free-floating “planktonic” cells, promoting the general theory of the unicellular lifestyle. However, in the late 1970s, Costerton et al. ( 1 ) demonstrated that groups of bacteria were embedded in a highly hydrated polysaccharide matrix that mediated adhesion to solid aquatic surfaces. Several years later, the same research team called these cellular communities “biofilms,” defined as a functionally heterogeneous aggregate of microcolonies or single cells encased in a matrix of self-produced extracellular polymeric molecules that could adhere either to organic, abiotic surfaces or to each other ( 2 ). Microbial biofilms can develop into highly organized structures containing channels in which water, nutrients, and metabolic waste can be transported. Adhesion to substrates or surfaces induces expression of a large number of genes, while cell aggregates in different regions in a biofilm exhibit different gene expression profiles that regulate biofilm development and maturation processes ( 3 , 4 ). A large amount of research since the 1980s has brought to light the theory that most, if not all, bacteria and fungi can form biofilms as a survival mechanism in hostile environments, providing protection from biotic and abiotic stresses ( 5 – 8 ). Prime candidates for cell attachment and biofilm growth are surfaces exposed to or containing moisture and some nutrients. Natural or man-made substrates for cell attachment and biofilm growth include river stones, oil and gas installations, ship hulls, water pipes, food-processing surfaces, contaminated surgical instruments, indwelling medical devices, human teeth, and infected wounds ( 9 – 11 ). In this chapter, we will present an overview of the life cycle of biofilms and their diversity, detection methods for biofilm development, and host immune responses to pathogens. We will then focus on current concepts in bacterial and fungal biofilm immune evasion mechanisms.